It looks like the rear of your trailer is the same as my 65 Caravel
. I had to replace the rear 10" or so of the floor due to water intrusion from the rear. I was able to do all of the replacement from underneath without removing any of the interior.
I believe the floor rot was accelerated by the portion of the hinged "trunk" lid that extends forward a couple of inches past the cross member. So, before I re-installed the trunk lid, I trimmed the forward sheet area of the hinge so that it no longer extends forward of the frame cross member.
During and after assembly I used loads of sealant. But if that fails for any reason, water coming in the rear will no longer be channeled toward the wood floor.
It took some thought to figure out how to do the job from the bottom, but it saved loads of time and work removing the fiberglass bathroom to get to the rear of the floor.
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